For the production of paper, chemical pulp must be delignified and bleached after cooking of pulp in a plurality of stages. Whereas in the past mainly elemental chlorine was used for delignifying and bleaching, bleaching sequences without elemental chlorine are now used in the ECF bleach. The bleaching sequence O-D0-EOP-D1-P is most frequently used for this purpose, O representing a delignification with oxygen under alkaline conditions, D0 and D1 designating first and second stages with chlorine dioxide as a bleaching and delignifying agent, EOP representing an alkaline extraction with addition of oxygen and hydrogen peroxide, P designating bleaching with hydrogen peroxide and each hyphen representing washing of the chemical pulp, for example by addition of water and filtration of the resulting suspension.
For the further development of the ECF bleach, a reduction of the amount of chlorine dioxide used is strived for in order further to reduce the formation of organochlorine compounds during the bleaching of the chemical pulp. Moreover, a simplification of the bleaching sequence with a reduction of the number of stages to only four stages and three washes in between is strived for in order to be able to carry out the ECF bleach with the same number of stages as bleaching of chemical pulp with elemental chlorine. However, the same whiteness of at least 89.5% ISO (according to PAPTAC standard E.1) and the same stability of the whiteness must be achieved as with the bleaching sequence O-D0-EOP-D1-P, and there must be no greater oxidative degradation of the chemical pulp.
U.S. Pat. No. 6,048,437 describes a process for delignifying and bleaching chemical pulp with a bleaching sequence O-DPcat-EOP-D1-P, in which, instead of a first stage D0 for delignifying and bleaching with chlorine dioxide, a stage DPcat is used in which the chlorine dioxide and hydrogen peroxide are used simultaneously in the presence of a molybdate or tungstate as a catalyst. Compared with the D0 stage of the conventional ECF bleach, the process permits a reduction of the required amount of chlorine dioxide but the desired whiteness of at least 89.5% ISO is not achieved with the amounts of chlorine dioxide and hydrogen peroxide used in U.S. Pat. No. 6,048,427. If the amounts of chlorine dioxide and hydrogen peroxide are increased, an undesirable oxidative degradation of the chemical pulp takes place, which is noticeable from a decrease in the viscosity of the chemical pulp, as is known from M. S. Manning et al., J. Pulp Paper Sci. 32 (2006) 58-62. Such an undesired degradation of the chemical pulp can be avoided only by introducing an additional stage Q of extraction of the chemical pulp with a metal ion chelating agent, corresponding to a bleaching sequence O-Q-DPcat-EOP-D1-P or Q-O-DPcat-EOP-D1-P. U.S. Pat. No. 6,048,437 also describes in comparative examples 13 and 14, that two stages D-Pcat or Pcat-D, in which Pcat represents bleaching with hydrogen peroxide in the presence of a molybdate or tungstate as a catalyst, provide substantially less bleaching effect compared with the DPcat stage.